1. Field of the Invention
The present invention relates to fibrous suspensions in paper making, and more specifically to drainage of fibrous suspensions.
2. Discussion of Background Information
Processes for drainage of fibrous suspensions are used either to increase the pulp density of the fibrous suspension, or to remove unwanted components from the fibrous suspension by washing. Both procedures are linked to each other, since drainage is necessary to the washing of a fibrous suspension. Washing processes generally start from relatively low initial pulp densities (e.g. 0.5-3% by weight). The extent of the effect of washing a fibrous suspension depends on additional parameters, i.e., not just the increase in pulp density.
Drainage processes may be implemented with various known devices for draining a fibrous suspension. The most significant known devices are wire fabric type presses and filter presses.
In wire fabric presses, the fibrous suspension is introduced between a wire fabric and roller, or between two wire fabrics. The fibrous suspension is compressed such that part of the water is extruded. The introduction of the suspension is accomplished by means of either a stuff chest or by means of a head box. The suspension runs out of a stuff chest into the region of the wire fabric(s). A headbox forms a jet for introduction of the suspension. For technical reasons, i.e., sealing problems, the stuff chest cannot be used with suspension densities which are too thin. For suspensions with thin densities, a headbox is essential. Although current processes have achieved high standards, it is often impossible to control the washing effect as desired.
For a fibrous suspension with a low initial pulp density (e.g. 0.5-2% by weight), a large amount of water must be removed therefrom, causing the suspension to behave Theologically similar to water. Therefore, only a slight pressure may be applied, e.g. by crushing the layer or by lateral spurting, if one wishes to avoid problems. On the other hand, too low a pressure causes the effectiveness of the process to be inadequate.
In fibrous suspensions, maximum removal of solids is in many cases not essential or is even disadvantageous. Often, a large number of requirements must be weighed against each other when determining the amount of solid removal.
U.S. Pat. No. 5,599,426, discloses a washing process such that the filtrate which appears is returned in several stages into the suspension (i.e. returned to the countercurrent). It is known that the countercurrent process increases the effectiveness of the separation processes but is associated with significant process technology and equipment expenditure.
WO 96/08600 discloses drainage processes in which friction forces are intentionally generated between two areas, moved relative to each other, which are strong enough that tiny rolls of thickened fiber pulp are formed thereby. A vacuum is applied to aid this effect. However, the type of processes disclosed in WO 96/08600 can be performed reliably only with relatively high pulp densities.